Western Quake 3 is where Smokin' Guns started. WQ3 was created by Iron Claw Interactive (if you search the username Spoon you will see their logo). Like dansh said, WQ3 is a mod for Quake 3 and required Quake 3 to be install in order to play.

Smokin Gun's Production picked up the mod, supported it for a few years, then released a stand alone version (SG1.0) using the Quake 3 engine. SG1.1 is using the IoQ3 engine (still Quake3, but supported by the good folks at IoQ3).

For more information, take a look at the manual that comes with Smokin Guns (C:/Program Files/Smokin' Guns/manual/index.html)